docs: def helical_turbulent_fd_Ju(Re, Di, Dc,roughness=0):
      r'''Calculates Darcy friction factor for a fluid flowing inside a curved 
      pipe such as a helical coil under turbulent conditions, using the method of 
      Ju et al. [1]_, also shown in [2]_.
          
      .. math::
          f_{curv} = f_{\text{str,turb}}\left[1 +0.11Re^{0.23}\left(\frac{D_i}
          {D_c}\right)^{0.14}\right]
          
      Parameters
      ----------
      Re : float
          Reynolds number with `D=Di`, [-]
      Di : float
          Inner diameter of the coil, [m]
      Dc : float
          Diameter of the helix/coil measured from the center of the tube on one
          side to the center of the tube on the other side, [m]
      roughness : float, optional
          Roughness of pipe wall [m]        
  
      Returns
      -------
      fd : float
          Darcy friction factor for a curved pipe [-]
  
      Notes
      -----
      Claimed to be valid for all turbulent conditions with :math:`De>11.6`.
      At very low curvatures, converges on the straight pipe result.
  
      Examples
      --------
      >>> helical_turbulent_fd_Ju(1E4, 0.01, .2)
      0.04945959480770937
  
      References
      ----------
      .. [1] Ju, Huaiming, Zhiyong Huang, Yuanhui Xu, Bing Duan, and Yu Yu. 
         "Hydraulic Performance of Small Bending Radius Helical Coil-Pipe." 
         Journal of Nuclear Science and Technology 38, no. 10 (October 1, 2001): 
         826-31. doi:10.1080/18811248.2001.9715102.
      .. [2] El-Genk, Mohamed S., and Timothy M. Schriener. "A Review and 
         Correlations for Convection Heat Transfer and Pressure Losses in 
         Toroidal and Helically Coiled Tubes." Heat Transfer Engineering 0, no. 0
         (June 7, 2016): 1-28. doi:10.1080/01457632.2016.1194693.
      '''
      fd = friction_factor(Re=Re, eD=roughness/Di)
      return fd*(1. + 0.11*Re**0.23*(Di/Dc)**0.14)